1. Field of the Invention
This invention relates to a process for making a circuit substrate, such as a package for mounting a semiconductor device or chip.
2. Description of the Related Art
Conventionally, several kinds of circuit substrate are known in which a circuit pattern is formed on a resin base and a plurality of metal leads are connected to the circuit pattern, such as a printed circuit board (PCB) or a package for mounting a semiconductor chip.
Several methods of forming such a circuit pattern on the resin base, are also known; for example, an electrodeposited copper foil is adhered to a surface of a resin base by a heat transfer or by an adhesive and then photoetched to form a fine circuit pattern. Alternatively, a plated film is formed on the surface of the resin base and then photoetched to form a fine circuit pattern.
According to the former method, however, if the surface of the resin base is provided with a recess for mounting a semiconductor chip, the electrodeposited copper foil can not be satisfactory adhered to a side or bottom wall of the above-mentioned recess, and thus a three-dimensional circuit pattern can not be easily formed on such a resin base.
According to the latter method, the surface of the resin base is usually electroless-plated to form a plated film thereon, and to ensure that the plated film is fixedly attached to the resin base, conventionally a surface thereof is roughened by an etching agent.
In a package for mounting a semiconductor chip or PCB, however, a material constituting the resin base must have a satisfactory heat resistance or acid resistance, or strength such as, for example, an engineering plastic including glass fillers, since the package for this purpose is usually highly sophisticated. On the other hand, the etching agent for roughening the surface of such a resin base must be an acid having a strong action, for example, nitric acid, sulfuric acid, fluoric acid, or the like. In other words, the material for the resin base must have etching characteristics such that it can be etched by these etching agents, and if such requirements are met, a highly sophisticated package can not be obtained. Therefore, it is difficult to appropriately select a material for the resin base, and the etching agent.
Also, if the resin base includes any stressed portion due to a molding process, cracks may occur or the etching speed may not be always the same when the resin base is etched, and therefore, the surface of the resin base will not be even and a thickness of the plated film formed thereon may be uneven.
In a package for mounting a semiconductor chip, advantageously metal parts, such as metal leads for connecting the circuit pattern to external circuits or lines, or a metal heat sink are integrally mounted within the resin base. But, if the resin base is dipped in a strong etching agent, these metal parts may be melted or damaged. In such a case, before etching, the surface of the resin base on which the metal parts are exposed is usually coated with a protective resist. But, the peripheral portions of metal parts on the resin base are unnecessarily covered with the protective resist, and a plated film having good adhesive characteristics can not be formed on these portions covered with the protective resist at a later stage.